Heat-sensitive adhesive sheet

ABSTRACT

A heat-sensitive adhesive sheet comprising a substrate having formed thereon an adhesive layer comprising a polymer composition (C) which comprises the following aqueous emulsion (A) and copolymer (B), wherein an adhesive force determined by the 180° peeling test method as specified in JIS Z 0237 at a temperature of 25° C. is 10 g/25 mm or less: Aqueous emulsion (A): An aqueous emulsion containing an alkyl (meth)acrylate-based polymer (a) having a glass transition temperature of −10° C. or below, which is obtained by emulsion-polymerizing radically polymerizable monomers containing as the main component an alkyl (meth)acrylate; and Copolymer (B): A water-soluble or water-dispersible copolymer (B) having a glass transition temperature of 20° C. or above, which is a copolymer obtained by copolymerizing an α,β-ethylenically unsaturated carboxylic acid with other radically polymerizable monomers, wherein a part or the entire amount of carboxyl groups in the copolymer is neutralized with a base having a boiling point of 110° C. or below.

TECHNICAL FIELD

This invention relates to an adhesive sheet. More particularly, itrelates to a heat-sensitive adhesive sheet which has non-adhesiveproperties or has very low adhesive properties at ordinary temperature,but exhibits adhesive properties upon heating and also exhibits adhesiveproperties by applying pressure.

BACKGROUND ART

In general, adhesive sheets with release papers (mold-release papers)covered on the adhesive surface thereof are subjected to storage,distribution, marketing, etc. Further, wind-up type products such asadhesive tapes use substrates having a back surface having beensubjected to a release treatment, so as to be capable of re-winding thetape when using the same. Since release papers become unnecessary afteradhering adhesive tapes onto adherends, release paper-free adhesivesheets are demanded from the reasons of resource saving and costreduction. Furthermore, if substrates, the back surface of which beingnot subjected to release treatment, can be used in adhesive tapes, it ispossible to simplify the production steps of the adhesive tapesubstrates and also decrease the production cost.

Known examples of release paper-free adhesive sheets includeheat-sensitive adhesive sheets coated with delayed-tack type adhesives(JP-B-62-21835 and JP-A-6-10084; the term “JP-A” as used herein means an“unexamined published Japanese patent application”, and the term “JP-B”as used herein means an “examined Japanese patent publication”). Thosepublications disclose delayed-tack type adhesives comprisingthermoplastic resins such as ethylene/vinyl acetate copolymers havingmixed therewith solid plasticizers such as dicyclohexyl phthalate. Thosepublications also describe that the adhesives have non-adhesiveproperties at ordinary temperature but exhibit adhesive force due tothat the plasticizers melt by heating.

However, these conventional delayed-tack type adhesives containing solidplasticizers have various problems as mentioned in (i) to (iv) below,and are applicable only to limited and specific purposes of use at thepresent stage.

(i) After crystallization of a solid plasticizer proceeds, adhesiveforce is lost. Therefore, once an adhesive sheet is peeled off from anadherend, such an adhesive tape cannot again be adhered thereto.

(ii) The heating temperature for exhibiting adhesive properties dependson the melting point of a solid plasticizer, which makes it impossibleto arbitrarily set the heating temperature.

(iii) After the crystallization proceeds, the adhesive becomes hard, andloses its flexibility. Therefore, if the adhesive tape is adhered to anadherend, and then the adherend is bent or vibration is applied to theadherend, there is a fear that the adhesive layer cannot follow theadherend and, as a result, peels off therefrom.

(iv) Where the substrate used in the adhesive sheet is made of woodfreepaper, etc., the plasticizer tends to ooze out onto the sheet surfacewhen the adhesive sheet is heated.

Therefore, an object of the present invention is to provide aheat-sensitive adhesive sheet which has non-adhesive properties or hasvery low adhesive properties at ordinary temperature and, therefore, canbe handled as marketed products as such even if release treatment to theadhesive layer (for example, a release paper or release treatment of theback surface of a substrate) is not always conducted, and on the otherhand, can exhibit adhesive properties when heating or when applyingpressure, thereby being effectively used as adhesive sheets or adhesivetapes.

Another object of the present invention is to provide a heat-sensitiveadhesive sheet having excellent qualities and free from theabove-mentioned problems (i) to (iv) in the conventional delayed-tacktype adhesives.

DISCLOSURE OF THE INVENTION

The present inventors have conducted extensive studies to solve theabove-mentioned problems. As a result, they have found that if aheat-sensitive adhesive sheet is formed by mixing an aqueous emulsioncontaining a specific alkyl (meth)acrylate-based polymer with acopolymer having carboxyl groups neutralized with a specific base toobtain a polymer composition and forming an adhesive layer comprisingthis polymer composition on a substrate, such an adhesive does not showadhesive properties at all or does not substantially show adhesiveproperties at ordinary temperature but exhibits adhesive properties whenheating or when applying pressure, and thus can effectively be used asan adhesive sheet.

Further, the present inventors have found that in the adhesive sheetthus obtained, the adhesive properties once exhibited are not lost andgood adhering state is maintained even if the adhesive sheet is adheredon a substrate and time passes; the adhesive layer does not become hardeven after the passage of time; and components in the adhesive layer donot ooze out onto the substrate even by heating. The present inventionhas been completed based on those findings.

Accordingly, the present invention provides a heat-sensitive adhesivesheet or a heat-sensitive adhesive tape, comprising a substrate havingformed thereon an adhesive layer comprising a polymer composition (C)which comprises the following aqueous emulsion (A) and copolymer (B),wherein an adhesive force determined by the 180° peeling test method asspecified in JIS Z 0237 at a temperature of 25° C. is 10 g/25 mm orless:

Aqueous emulsion (A): An aqueous emulsion containing an alkyl(meth)acrylate-based polymer (a) having a glass transition temperatureof −10° C. or below, which is obtained by emulsion-polymerizingradically polymerizable monomers containing as the main component analkyl (meth)acrylate; and

Copolymer (B): A water-soluble or water-dispersible copolymer (B) havinga glass transition temperature of 20° C. or above, which is a copolymerobtained by copolymerizing an α,β-ethylenically unsaturated carboxylicacid with other radically polymerizable monomers, wherein a part or theentire amount of carboxyl groups in the copolymer is neutralized with abase having a boiling point of 110° C. or below.

Further, the present invention is a heat-sensitive adhesive sheet,characterized in that it is a heat-sensitive adhesive sheet having on asubstrate an adhesive layer comprising a polymer composition emulsion(C) obtained by emulsion polymerizing a radically polymerizable monomercomprising as a main component, an alkyl (meth)acrylate forming an alkyl(meth)acrylate-based polymer (a) having a glass transition temperatureof −10° C. or less, in an aqueous medium in the presence of awater-soluble or water-dispersible copolymer (B) having a glasstransition temperature of 20° C. or more, which is a copolymer having anacid value of 30-260 mgKOH/g obtained by copolymerizing anα,β-ethylenically unsaturated carboxylic acid with other radicallypolymerizable monomers, wherein a part or the entire amount of carboxylgroups in the copolymer is neutralized with a base having a boilingpoint of 110° C. or less; and that an adhesive force determined by the180° peeling test method as specified in JIS Z 0237 at a temperature of25° C. is 10 g/25 mm or less

The heat-sensitive adhesive sheet of the present invention as describedabove can be used without subjecting any release treatment to theadhesive layer (that is, without using a release paper or withoutsubjecting release treatment to the back surface of the substrate).Further, the sheet can also be used in the state that the releasetreatment has been applied, if desired.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail below.

The term “heat-sensitive adhesive sheet” as used herein means one havingan adhesive layer comprising a polymer composition which comprises theabove-mentioned aqueous emulsion (A) and copolymer (B), formed on asubstrate. The substrate in such a heat-sensitive adhesive sheet is notparticularly limited in shape, size, form, material, etc., and any onemay be used. Accordingly, the “heat-sensitive adhesive sheet” of thepresent invention means all of heat-sensitive adhesive sheets havingsubstrates in the form of a sheet, heat-sensitive adhesive tapes havingsubstrates in the form of a tape, those having substrates in the form ofa plate and those having substrates in the form of shapes or forms otherthan the above-mentioned types.

Further, the heat-sensitive adhesive sheet of the present invention maybe one having an adhesive layer on one surface of the substrate, onehaving an adhesive layer on both surfaces of a substrate, or one havingan adhesive layer on portions other than front surface/back surface of asubstrate when the substrate has a thickness to a certain extent.

The aqueous emulsion (A) used in the adhesive layer of theheat-sensitive adhesive sheet of the present invention is an aqueousemulsion comprising an alkyl (meth)acrylate-based polymer having a glasstransition temperature (hereinafter simply referred to as “Tg”) of −10°C. or below, which is obtained by emulsion-polymerizing radicallypolymerizable monomers comprising as the main component an alkyl(meth)acrylate. More preferable Tg of the alkyl (meth)acrylate-basedpolymer contained in the aqueous emulsion (A) is −30° C. or below. Ifthe Tg of the alkyl (meth)acrylate-based polymer exceeds −10° C.,sufficient adhesive force does not exhibit even when heating or applyingpressure, and the desired heat-sensitive adhesive sheet is not obtained.

An alkyl (meth)acrylate-based polymer having Tg of −10° C. or below willhereinafter be referred to as an “alkyl (meth)acrylate-based polymer(a)”.

Tg of the polymer in the present invention is calculated from thefollowing calculation formula (1):

1/Tg=ΔW(a)/Tg(a)+ΔW(b)/Tg(b)+ΔW(c)/Tg(c)  (1)

In the above formula:

Tg=Tg of the polymer;

W(a)=weight ratio of structural unit comprising monomer (a) in thepolymer;

W(b)=weight ratio of structural unit comprising monomer (b) in thepolymer;

W(c)=weight ratio of structural unit comprising monomer (c) in thepolymer;

Tg(a)=glass transition temperature of homopolymer of the monomer (a);

Tg(b)=glass transition temperature of homopolymer of the monomer (b);and

Tg(c)=glass transition temperature of homopolymer of the monomer (c).

The aqueous emulsion (A) preferably used is one obtained byemulsion-polymerizing an alkyl (meth)acrylate used as the main component(preferably 60 to 100% by weight) optionally with a small amount(preferably 40% by weight or less) of other radically polymerizablemonomers copolymerizable with the alkyl (meth)acrylate (hereinaftersometimes simply referred to as “copolymerizable monomers”) in anaqueous medium, as will be described hereinafter.

The alkyl (meth)acrylate used in the production of the aqueous emulsion(A) is preferably an alkyl (meth)acrylate having an alkyl group of 1 to9 carbon atoms, and examples thereof include methyl (meth)acrylate,ethyl (meth)acrylate, isopropyl (meth)acrylate, n-propyl (meth)acrylate,n-butyl (meth)acrylate, isobutyl (meth)acrylate, hexyl (meth)acrylate,cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl(meth)acrylate, isooctyl (meth)acrylate, n-nonyl (meth)acrylate andisononyl (meth)acrylate. One kind or two kinds or more of those alkyl(meth)acrylates can be used. Further preferably alkyl (meth)acrylateused in the production of the aqueous emulsion (A)is an alkyl(meth)acrylate having analkyl group of 4 to 9 carbon atoms.

Examples of the copolymerizable monomer include aromatic vinyl monomerssuch as styrene, α-methylstyrene and vinyltoluene; unsaturatedcarboxylic acids such as (meth)acrylic acid, crotonic acid, cinnamicacid, itaconic acid, fumaric acid and maleic acid; monoalkyl esters ofunsaturated dicarboxylic acids such as monoethyl itaconate, monobutylfumarate and monobutyl maleate; hydroxylated vinyl monomers such as2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,polyethylene glycol (meth)acrylate and polypropylene glycol(meth)acrylate; (meth)acrylonitrile, vinyl acetate, (meth)acrylamide,N-methylolacrylamide, glycidyl methacrylate, vinyl acetate, vinylchloride and vinylidene chloride. One kind or two kinds or more of thosecopolymerizable monomers can be used.

Where the above-described copolymerizable monomer is employed, it ispreferable that the amount of the copolymerizable monomer used is 40% byweight or less as described above based on the total weight of theradically polymerizable monomers used for the production of the aqueousemulsion (A). If the amount of the copolymerizable monomer used exceeds40% by weight, adhesive properties of the alkyl (meth)acrylate-basedpolymer obtained tend to be insufficient. As a result, even if aheat-sensitive adhesive sheet having an adhesive layer containing suchan aqueous emulsion (A) is heated, it becomes difficult to exhibitadhesive properties.

The production method of the aqueous emulsion (A) is not particularlylimited, and can be conducted in the same manner as in the conventionalemulsion-polymerization. That is, the aqueous emulsion (A) can beobtained by uniformly dispersing the above-mentioned alkyl(meth)acrylate optionally together with the above-mentionedcopolymerizable monomer in an aqueous medium using a conventionalprotective colloid or a surfactant as an emulsifier and then conductingemulsion polymerization using a polymerization initiator.

Examples of the polymerization initiator include persulfate typepolymerization initiators such as potassium persulfate and ammoniumpersulfate; azo type polymerization initiators such as2,2′-azobisisobutyronitrile and 2,2′-azobis(2-methylbutyronitrile);organic peroxides such as benzoyl peroxide and lauroyl peroxide; andredox type polymerization initiators comprising a combination of anoxidizing agent comprising organic perhydroxides (cumene hydroperoxide,tert-butyl hydroperoxide, diisopropylbenzene hydroperoxide, etc.) and areducing agent (Rongalite, sodium hydrogensulfite, ascorbic acid, etc.).Preferable amount of such a polymerization initiator used is 0.1 to 5%by weight based on the total weight of the monomers.

Examples of surfactants usable as the emulsifier include anionicsurfactants such as sodium higher alcohol sulfates, sodiumalkylbenzenesulfonates, sodium dialkyl succinate sulfonates and sodiumalkyl diphenyl ether disulfonates; and nonionic surfactants such aspolyoxyethylene alkyl allyl ethers and polyoxyethyelne/polyoxypropyleneblock copolymers. Further, reactive anionic surfactants such as sodiumallylalkylsulfonates, alkylallylsulfosuccinates and polyoxyethylenealkylallylglycerol ether sulfates may be used.

The present invention can also use the copolymer (B) as will bedescribed hereinafter as a polymeric emulsifier. That is, the polymercomposition (C) comprising the aqueous emulsion (A) and the copolymer(B) can also be obtained by emulsion polymerizing radicallypolymerizable monomers containing as the main component an alkyl(meth)acrylate capable of providing the above-mentioned alkyl(meth)acryalte-based polymer (a) in an aqueous medium in the presence ofthe copolymer (B). According to this production method, since thepolymer composition (C) can be obtained even if a low molecular weightsurfactant is not used or the amount of the surfactant is decreased to asmall amount, an adhesive having excellent water resistance tends to beeasily obtained.

In the emulsion polymerization for the production of the aqueousemulsion (A), a chain transfer agent may be used to control themolecular weight of the alkyl (meth)acrylate-based polymer product.Examples of the chain transfer agent include mercaptoacetic acid,mercaptopropionic acid, 2-propanethiol, 1-butanethiol,2-methyl-2-propanethiol, 2-mercaptoethanol, ethyl mercaptoacetate,thiophenol, 2-naphthalenethiol, dodecyl mercaptan and thioglycerol.

The polymerization temperature in producing the aqueous emulsion (A) ispreferably about 0 to 150° C., and more preferably 20 to 90° C. Thepolymerization in such a case is preferably 1 to 24 hours, and morepreferably 3 to 8 hours.

The content (solid content) of the alkyl (meth)acrylate-based polymer(a) in the aqueous emulsion (A) obtained is preferably 5 to 80% byweight, and more preferably 30 to 70% by weight, from the point of easeof handling properties.

The copolymer (B) used together with the above-mentioned aqueousemulsion (A) in the adhesive layer of the heat-sensitive adhesive sheetof the present invention is a water-soluble or water-dispersiblecopolymer having Tg of 20° C. or above, which is obtained bycopolymerizing an α,β-ethylenically unsaturated carboxylic acid withother radically polymerizable monomers (hereinafter sometimes referredto as “copolymerizable monomers”), wherein a part or the entire amountof the carboxyl groups in the copolymer obtained above (hereinafterreferred to as a “copolymer before neutralization”) is neutralized witha base having a boiling point of 110° C. or below.

If Tg of the copolymer (B) is lower than 20° C., the adhesive layercomprising the polymer composition (C) obtained by mixing the copolymer(B) with the above-mentioned aqueous emulsion (A) fails to shownon-adhesive properties or low adhesive properties at room temperature.That is, the adhesive force at 25° C. determined by the 180° peelingtest as specified in JIS Z 0237 exceeds 10 g/25 mm and, as a result, theobject of the present invention cannot be achieved. Further, if Tg ofthe copolymer (B) is extremely high, heating temperature for exhibitingthe adhesive properties becomes high. As a result, the workability andheat efficiency during adhering operation of the sheet decrease. Inaddition, the substrate of the heat-sensitive adhesive sheet tends tomodify due to high temperature when heating. Therefore, consideringthose points, preferable Tg of the copolymer (B) is 30 to 200° C., andmore preferable Tg is 70 to 150° C.

Examples of the α,β-ethylenically unsaturated carboxylic acid used forproducing the copolymer before neutralization include acrylic acid,methacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconicacid, citraconic acid and maleic anhydride. Preferred is acrylic acidand/or methacrylic acid therefor. Preferable amount of the α,β-ethylenicunsaturated carboxylic acid used is such an amount that an acid value ofthe copolymer prior to the neutralization with the base (i.e., thecopolymer before neutralization), obtained by copolymerizing theα,β-ethylenically unsaturated carboxylic acid with other radicallypolymerizable monomers is 30 to 260 mgKOH per 1 g of the copolymerbefore neutralization. If the acid value of the copolymer beforeneutralization is less than 30 mgKOH per 1 g of the copolymer beforeneutralization, it is difficult to obtain a copolymer which can welldissolve or disperse in water even if neutralizing the same with a base.On the other hand, if the acid value thereof exceeds 260 mgKOH/g, thecopolymer obtained has poor water resistance.

The amount of the α,β-ethylenically unsaturated carboxylic acid used togive the acid value as specified above varies depending on the type ofthe unsaturated carboxylic acid used. However, the α,β-ethylenicallyunsaturated carboxylic acid is generally used in an amount of 3 to 40%by weight based on the total weight of the monomers used in theproduction of the copolymer before neutralization. Examples of the othercopolymerizable monomer used together with the above-mentionedα,β-ethylenically unsaturated carboxylic acid for producing thecopolymer (B) (i.e., the copolymer before neutralization) include(meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate,propyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate,tert-butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate,octyl (meth)acrylate, 2-ethylhexyl (meth) acrylate, cyclohexyl (meth)acrylate, 2-hydroxyethyl (meth) acrylate, 2-hydroxybutyl (meth)acrylate,polyalkyleneglycol (meth)acrylate, glycidyl (meth)acrylate,dimethylaminoethyl (meth)acrylate and perfluoroalkyl (meth)acrylate;aromatic vinyl compounds such as styrene, vinyltoluene andα-methylstyrene; (meth)acrylonitrile, vinyl acetate, vinyl propionate,N-methylol(meth)acrylamide, vinyl chloride, vinyl fluoride, vinylidenechloride, vinylidene fluoride, ethylene and trichloroethylene. One kindor two kinds or more of these monomers can be used. Of those, alkyl(meth)acrylates having alkyl groups of 1 to 4 carbon atoms arepreferably used.

The type and the use proportion of the α,β-ethylenically unsaturatedcarboxylic acid and other copolymerizable monomers are selected based onthe above-mentioned calculation formula (1) such that Tg of the polymerobtained is 20° C. or above.

As a polymerization method to obtain the above-mentioned copolymerbefore neutralization, conventional polymerization methods such as amethod of using a radical polymerization initiator or a method byirradiation with radiation can be used. A method of using a radicalpolymerization initiator is preferable from the points of ease ofpolymerization operation and ease of molecular weight control of thealkyl (meth)acrylate-based polymer obtained. Further, the polymerizationmethod includes solution polymerization method, suspensionpolymerization method, emulsion polymerization method, bulkpolymerization method and precipitation polymerization method. Of those,a solution polymerization method wherein the polymerization reaction isconducted in an organic solvent is preferably employed.

Examples of the organic solvent preferably used in the above-describedsolution polymerization include ketone solvents such as acetone, methylethyl ketone and methyl isobutyl ketone; acetate solvents such as ethylacetate and butyl acetate; aromatic hydrocarbon solvents such asbenzene, toluene and xylene; aliphatic hydrocarbon solvents such ascyclohexane, hexane and heptane; and alcoholic solvents such asmethanol, ethanol, isopropanol, methyl cellosolve, ethyl cellosolve,n-butyl cellosolve, ethylene glycol, propylene glycol,trimethylolpropane and glycerol. Those organic solvents may be usedalone or as a mixture of two or more thereof. Of those, methyl ethylketone and/or isopropyl alcohol are more preferably used as the organicsolvent.

As the radical polymerization initiator used in the solutionpolymerization, any one generally used in radical polymerization can beused. For example, the same organic peroxides, inorganic peroxides orazo compounds as those exemplified above as the polymerization initiatorfor obtaining the aqueous emulsion (A) can be used.

In the above-mentioned solution polymerization, a chain transfer agentmay be added to the polymerization system in order to control themolecular weight of the copolymer before neutralization. As the chaintransfer agent in such a case, chain transfer agents having a mercaptogroup similar to the chain transfer agent described above regarding theproduction of the aqueous emulsion (A) are preferably used.

The polymerization temperature for obtaining the copolymer beforeneutralization is preferably about 10 to 150° C., and more preferably 60to 100° C. Further, the polymerization time is preferably 1 to 100hours, and more preferably 3 to 10 hours.

It is preferable that the copolymer before neutralization has a numberaverage molecular weight of 1,000 to 500,000. If the number averagemolecular weight of the copolymer before neutralization is less than1,000, water resistance or moisture resistance of the adhesive layer inthe heat-sensitive adhesive sheet tends to be poor. On the other hand,if it exceeds 500,000, it becomes difficult to produce the copolymerbefore neutralization per se due to the high viscosity.

A part of or the entire carboxyl groups in the copolymer beforeneutralization obtained by the above polymerization is then neutralizedwith a base having a boiling point of 110° C. or below to convert to thecopolymer (B) used in the present invention.

If the boiling point of the base used for the neutralization exceeds110° C., the adhesive layer of the obtained heat-sensitive adhesivesheet has poor water resistance.

Specific examples of the base having a boiling point of 110° C. or belowused in the neutralization of the carboxyl groups in the copolymerbefore neutralization include ammonia, methylamine, ethylamine,propylamine, isopropylamine, butylamine, amylamine, dimethylamine,diethylamine, dipropylamine, diisopropylamine, trimethylamine,triethylamine and allylamine. Of those, ammonia is preferably used.

The neutralization amount of carboxyl groups in the copolymer beforeneutralization is preferably 30 mol % or more, and more preferably 50mol % or more, of the carboxyl groups. If the neutralization amount ofthe carboxyl groups in the copolymer before neutralization is less then30 mol %, surface activity, water solubility, water dispersibility,etc., of the copolymer (B) decrease, so that when the aqueous emulsion(A) and the copolymer (B) are mixed, it becomes difficult for thecopolymer (B) to uniformly dissolve or disperse in the aqueous emulsion(A). As a result, the adhesive layer formed from such a mixture tend tohave poor physical properties (for example, development property ofadhesive force when heating, adhesive force, or the like).

To neutralize the carboxyl groups in the copolymer beforeneutralization, it is preferable that the copolymer beforeneutralization obtained by, for example, the above-mentioned solutionpolymerization is not taken up from the reaction medium but allowed toremain therein as such (in particular, in a state being dissolved in anorganic solvent) and then a base for neutralization is added thereto,since this procedure can be easily performed. It is still preferable toadd the base for neutralization in the form of an aqueous solution,since the carboxyl groups can be smoothly neutralized thereby.

That is to say, it is preferable to employ a process which comprisesproducing the copolymer before neutralization by the solutionpolymerization; adding a base for neutralization thereto to therebyneutralize the carboxyl groups in the copolymer before neutralization,thus forming a copolymer (B) in the reaction medium; removing theorganic solvent existing in the reaction system by an appropriateprocedure (for example, under reduced pressure) to obtain an aqueoussolution or an aqueous dispersion of the copolymer (B); then mixing theaqueous solution or aqueous dispersion as such with an aqueous emulsion(A) to obtain an aqueous polymer composition; and applying the aqueouspolymer composition to a substrate to thereby form an adhesive layer,since this process can be easily carried out and have of only a smallnumber of steps. In this process, it is preferable from the viewpointsof the handling properties and the formation of the adhesive layer tocontrol the content of the copolymer (B) (in terms of solid content) inthe aqueous solution or aqueous dispersion thereof to about 10 to 80% byweight before mixing with the aqueous emulsion (A).

Of course, the copolymer (B) can be separated and recovered from thereaction system after completion of the neutralization and then mixedwith the aqueous emulsion (A).

As described above, it is also possible in the present invention toemulsion-polymerize in an aqueous medium a radically polymerizablemonomer capable of forming an alkyl (meth)acrylate-based polymer (a) inthe presence of the copolymer (B). In this case, the aqueous solution oraqueous dispersion of the copolymer (B) obtained by the above-mentionedmethod is employed as the polymerization medium. It is preferable thatthe concentration of the copolymer (B) in the aqueous solution oraqueous dispersion thereof to be used as the polymerization mediumranges from 10 to 60% by weight.

In the polymer composition (C) comprising the above-mentioned aqueousemulsion (A) and copolymer (B), the weight ratio of the copolymer (B)(in terms of solid content) to the alkyl (meth)acrylate-based polymer(a) (in terms of solid content) is preferably from 5/95 to 80/20, andmore preferably from 10/90 to 60/40. [The polymer composition (C)obtained by mixing the aqueous emulsion (A) with the copolymer (B) willbe sometimes referred to as the “adhesive composition”.] When thecontent of the copolymer (B) is less than 5% by weight based on thesolid content in the adhesive composition, the emulsion particles of theaqueous emulsion (A) cannot be sufficiently coated with the copolymer(B). Thus, the adhesive layer of the resulting adhesive sheet shows anadhesive strength exceeding 10 g/25 mm at ordinary temperature (25° C.).Such an adhesive sheet is excluded from the scope of the presentinvention which relates to adhesive sheets requiring no releasetreatment (i.e., treatment with release paper or release agents). Whenthe content of the copolymer (B) in the adhesive composition exceeds 80%by weight based on the solid content in the adhesive composition, on theother hand, it is frequently observed that the resulting adhesivecomposition exhibits only poor adhesion performance upon heating orunder elevated pressure due to its excessively high Tg.

The above-mentioned adhesive composition to be used in the adhesivelayer of the heat-sensitive adhesive sheet of the present invention mayfurther contain, depending on the purpose of use thereof, one or moreadditives commonly used in adhesives, for example, defoaming agents,surfactants, mildewproofing agents, perfumes, neutralizers, tackifiers,thickeners, leveling controlling agents, antifreezing agents, foamingagents, antioxidants, UV absorbers, reinforcing agents, fillers,pigments, fluorescent brighteners, antistatic agents, antiblockingagents, flame-retardants, crosslinking agents, plasticizers, lubricants,organic solvents and coloring matters.

In the heat-sensitive adhesive sheet of the present invention, thesubstrate is not particularly limited, an appropriate one can be useddepending on the purpose of use of the heat-sensitive adhesive sheet.Examples thereof include films, sheets and plates made of fabrics,papers, leathers, woods, metals, glass and various plastics, and foamedplastic sheets. Examples of the substrates made of the above-mentionedplastics include those of polyester, polyamide, vinyl chloride-basedpolymers, polyethylene, polypropylene and polyurethane. The substratemay have an arbitrary shape, for example, a continuous one or one cutinto a definite size (for example, having been cut into a square,circular or ellipse piece).

The heat-sensitive adhesive sheet of the present invention can beproduced by applying an aqueous adhesive composition onto one or bothsides of the above-mentioned substrate and then drying the same in anappropriate manner. The adhesive composition may be applied onto thesubstrate by an arbitrary method without limitation, for example, rollcoating, spray coating, casting, doctor blade coating or brush coating.To dry the adhesive composition applied onto the substrate, it isnecessary to heat the composition at such a temperature that theadhesive layer in the finally obtained adhesive sheet would not exhibitany adhesive properties at ordinary temperature. The adhesivecomposition is dried at a temperature not higher than the Tg of thecopolymer (B) contained therein. It is more preferable to dry theadhesive composition at a temperature lower by 10 to 20° C. than the Tgof the copolymer (B).

In the heat-sensitive adhesive sheet of the present invention, thethickness of the adhesive layer can be appropriately controlleddepending on the purpose of use, etc. Generally, It is preferable thatthe thickness is about 1 μm to 1 mm.

The heat-sensitive adhesive sheet of the present invention has anadhesive strength determined by the 180° peeling test method asspecified in JIS Z 0237 at a temperature of 25° C. of 10 g/25 mm orless, so that adhesive properties are not shown at ordinary temperatureor the adhesive force is very small at ordinary temperature.Accordingly, it can be optionally packed and stored, distributed andsold as such, similar to usual products having no adhesive properties,without subjecting the adhesive layer to any release treatment. Ofcourse, if necessary, release treatment may be applied, for example,where the heat-sensitive adhesive sheet has an adhesive strength of 3 to10 g/25 mm at 25° C., depending on the purpose use of the adhesivesheet.

In adhering the heat-sensitive adhesive sheet of the present inventionto an adherend, the heat-sensitive adhesive sheet can be adhered to anadherend by heating either or both of the adhesive layer of theheat-sensitive adhesive sheet and the adherend. The heating temperaturefor exhibiting the adhesive properties in the adhesive layer depends onthe Tg of the copolymer (B) contained in the adhesive composition. Inusual, the heating temperature is a temperature at least 10° C., andpreferably at least 30° C., higher than the Tg.

The heat-sensitive adhesive sheet of the present invention once adheredto the adherend is well adhere to the adherend without losing itsadhesive properties even after cooling to ordinary temperature, and doesnot peel off from the adherend spontaneously. When the heat-sensitiveadhesive sheet of the present invention is desired to separate from theadherend, it can be easily peeled off therefrom by merely pulling withhand similar to common adhesive sheets. The adhesive layer of theheat-sensitive adhesive sheet thus peeled off still sustains itsadhesive properties. Therefore, the heat-sensitive adhesive sheet can beadhered again to the same adherend or another one as such.

Where the adhesive properties are exhibited in the adhesive layer of theheat-sensitive adhesive sheet of the present invention by applyingpressure, the heat-sensitive adhesive sheet can be adhered to anadherend by applying a pressing force in the same degree of a writingpressure when writing with a ball-point pen, to the heat-sensitiveadhesive sheet.

Even where the adhesive properties are exhibited in the adhesive layerof the heat-sensitive adhesive sheet by applying pressure and the sheetis adhered to an adherend, the adhesive force once exhibited is not losteven after removing the pressure, similar to the case where the adhesiveproperties are exhibited by heating.

Further, the adhesive properties may be exhibited in the heat-sensitiveadhesive sheet of the present invention by heating and applying pressuretogether.

The adherend to which the heat-sensitive adhesive sheet of the presentinvention is adhered is not particularly limited in its material ortype. For example, the heat-sensitive adhesive sheet of the presentinvention can be well adhered to fabrics, papers, leathers, woods,metals, glasses, concretes, ceramics, polyester, polypropylenes,polyvinyl chlorides, polyurethanes, polycarbonates, polyamides,polyimides, styrene-based polymers and rubbers.

The present invention is explained specifically by referring to thefollowing Synthesis Examples, Examples and Comparative Examples. Unlessotherwise indicated, all “part” and “%” given in these examples are byweight. Further, glass transition temperature (Tg) of polymers, andadhesive force, holding power, probe tack and water resistance ofadhesive sheets were measured respectively by the following methods.

(1) Adhesive Force of Adhesive Sheet:

An adhesive composition was applied onto a polyester film having athickness of 50 μm at a dry coating thickness of 20 to 30 μm and thendried at 70° C. for 5 minutes to prepare an adhesive sheet. This sheetwas cut into test pieces having a width of 25 mm and a length of about250 mm. After not heating (25° C.), heating at 100° C. for 2 minutes orheating at 140° C. for 2 minutes, those test pieces were placed on apolished stainless test plate and adhered thereto by pressing down witha 2 kg rubber roller reciprocally. After 30 minutes, 180° peelingadhesive force to the test plate was obtained at a tensile speed of 300mm/min. It was measured in accordance with adhesive force of JIS Z 0237.

(2) Holding Power of Adhesive Sheet:

It was measured in accordance with holding power of JIS Z 0237. Namely,the same adhesive sheet as in the case of the adhesion test was preparedand cut into pieces having a width of 25 mm in width and a length ofabout 150 mm to obtain test pieces. After (i) not conducting heattreatment (25° C.), (ii) heating at 100° C. for 2 minutes or (iii)heating at 140° C. for 2 minutes, those test pieces were adhered to astainless test plate such that each test piece came in contact with theplate in an area of 25 mm×25 mm, and then pressed thereto by pressingdown with a 2 kg rubber roller reciprocally. After 30 minutes, a 1 kgload was applied thereon at 40° C. and the time until peeling off wasmeasured. When a test piece was held for 3 hours, the slippage distancewas measured. When a test piece peeled off immediately after loading,the holding time was referred to as “0 minute”.

(3) Probe Tack of Adhesive Sheet:

Measurement was performed in accordance with the probe tack test asdefined in JIS Z 0237. Namely, the same adhesive sheet as in the case ofthe adhesive force test was prepared and cut into pieces having a widthof about 20 mm and a length about 20 mm to obtain test pieces. Aftersubjecting those test pieces to the same treatments as in (i) to (iii)described in the above (2), the measurement was carried out under thetest conditions of columnar probe=5 mm diameter; contact speed andpeeling speed=0.2 cm/sec; and contact load=0.98±0.01 N/cm.

(4) Water Resistance of Adhesive Sheet:

The same adhesive sheet as in the case of the adhesive strength test wasprepared and immersed in water at 20° C. for 1 hour. Whitened state ofthe pieces was visually observed and evaluated as follows: Not whitened:O; and Whitened: x.

SYNTHESIS EXAMPLE 1 Preparation of Aqueous Emulsion (A1)

(1) A monomer mixture was prepared by mixing 90 parts of 2-ethylhexylacrylate, 8 parts of methyl methacrylate, 1 part of 2-hydroxyethylmethacrylate and 1 part of methacrylic acid.

(2) 100 parts of deionized water and 0.5 part of sodium lauryl sulfatewere supplied into a flask provided with a stirrer, a thermometer, acondenser and a nitrogen inlet tube. After heating to 60° C. in anitrogen atmosphere, 5 parts of an aqueous solution of t-butylhydroperoxide and 10 parts of a 10% aqueous solution of Rongalite(formaldehyde sodium sulfoxylate dihydrate) were added thereto. Further,100 parts of the monomer mixture prepared in the above (1) was addeddropwise thereto over 3 hours. After completion of the addition, theresulting mixture was reacted for additional 2 hours at the sametemperature to thereby complete the polymerization, thus obtaining anaqueous emulsion (A) (solid content: 50%) containing an alkyl(meth)acrylate-based polymer (this emulsion is hereafter referred to as“Aqueous Emulsion (A1)”).

SYNTHESIS EXAMPLE 2 Preparation of Aqueous Emulsion (A2)

(1) A monomer mixture was prepared by mixing 24 parts of butyl acrylate,70 parts of 2-ethylhexyl acrylate, 4.9 parts of acrylonitrile, 1 part ofacrylic acid and 0.1 part of N-methylolacrylamide.

(2) To the monomer mixture prepared in the above (1) were added 1 partof sodium polyoxyethylene nonylphenyl ether sulfate and 40 parts ofdeionized water. The resulting mixture was emulsified with a three-onemortar to prepare an aqueous emulsified dispersion.

(3) 60 parts of deionized water and 0.2 part of sodium carbonate (aneutralizing agent) were fed into the same type of a flask as used inSynthesis Example 1 (2). Under stirring in a nitrogen atmosphere whilemaintaining the inner temperature at 70° C., 10 parts of an aqueoussolution of ammonium persulfate (ammonium persulfate/deionized water=0.3part/0.7 part) and 200 parts of the aqueous emulsified dispersionprepared in the above (2) were added dropwise thereto over 3 hours.After completion of the addition, the resulting mixture was reacted foradditional 2 hours at the same temperature to thereby complete thepolymerization, thus obtaining an aqueous emulsion (A) (solid content:50%) containing an alkyl (meth)acrylate-based polymer (this emulsion ishereinafter referred to as “Aqueous Emulsion (A2)”.

SYNTHESIS EXAMPLE 3 Preparation of Aqueous Emulsion (A3)

Emulsion polymerization was conducted in the same manner as in SynthesisExample 2 except for using a monomer mixture of 95 parts of isononylacrylate, 4 parts of methyl methacrylate, 0.8 parts of methacrylic acidand 0.2 parts of N-methylolacrylamide, and using 1 part of sodium laurylsulfate as an emulsifier, thus obtaining an aqueous emulsion (A) (solidcontent: 50%) containing an alkyl (meth)acrylate-based polymer (thisemulsion is hereinafter referred to as “Aqueous Emulsion (A3)”.

SYNTHESIS EXAMPLE 4 Preparation of Aqueous Emulsion (A4)

Emulsion polymerization was conducted in the same manner as in SynthesisExample 1 except for using a monomer mixture of 95 parts of butylacrylate, 4 parts of acrylonitrile and 1 part of acrylic acid, thusotaining an aqueous emulsion (A) (solid content: 50%) containing analkyl (meth)acrylate-based polymer (this emulsion is hereinafterreferred to as “Aqueous Emulsion (A4)”.

SYNTHESIS EXAMPLE 5 Preparation of Aqueous Emulsion (A5)

Emulsion polymerization was conducted in the same manner as in SynthesisExample 1 except for using a monomer mixture of 98 parts of 2-ethylhexylacrylate, 1 part of methacrylic acid and 1 part of 2-hydroxyethylmethacrylate, thus obtaining an aqueous emulsion (A) (solid content:50%) containing an alkyl (meth)acrylate-based polymer (this emulsion ishereinafter referred to as “Aqueous Emulsion (A5)”.

The contents of the respective aqueous emulsions (A) obtained in theabove Synthesis Examples 1 to 5 [solid content of the aqueous emulsionA, and monomer composition and Tg of alkyl (meth)acrylate-based polymercontained in the aqueous emulsion (A)] are shown in the following Table2.

In the following Tables 2 and 3, monomers are expressed in abbreviation.The abbreviation and its content are as shown in Table 1 below.

TABLE 1 Abbreviation Kind of Monomer BMA Butyl methacrylate MMA Methylmethacrylate AN Acrylonitrile HEMA 2-Hydroxyethyl methacrylate MAAMethacrylic acid ST Styrene AA Acrylic acid AEH 2-Ethylhexyl acrylate BAButyl acrylate EA Ethyl acrylate INA Isononyl acrylate NMAMN-Methylolacrylamide

TABLE 2 [Contents of aqueous emulsion (A)] SolidAlkyl(methacrylate)-based polymer content Monomer composition Tg¹⁾Symbol (%) (weight ratio) (° C.) Synthesis A1 50 AEH/MMA/HEMA/MAA =−60.2 Example 1 90/8/1/1 Synthesis A2 50 BA/AEH/AN/AA/NMAM = −60.2Example 2 24/70/4.9/1/0.1 Synthesis A3 50 INA/MMA/MAA/NMAM = −77.1Example 3 95/4/1/0.2 Synthesis A4 50 BA/AN/AA = 95/4/1 −51.2 Example 4Synthesis A5 50 AEH/HEMA/MAA = 90/1/1 −68.2 Example 5 ¹⁾Caculated.

SYNTHESIS EXAMPLE 6 Preparation of Aqueous Solution of Copolymer (B1)

(1) 0.5 Part of 2,2′-azobisisobutyronitrile was dissolved in a liquidmixture of 20 parts of butyl methacrylate, 10 parts of methylmethacrylate, 35 parts of acrylonitrile, 10 parts of 2-hydroxyethylmethacrylate, 25 parts of methacrylic acid and 100 parts of methyl ethylketone, and the resulting solution was supplied into a flask equippedwith a stirrer, a condenser, a thermometer and a nitrogen inlet tube.After heating at 80° C. in a nitrogen atmosphere for 4 hours, 0.5 partof additional 2,2′-azobisisobutyronitrile was added thereto. Theresulting mixture was heated at the same temperature for 5 hours,thereby obtaining a methyl ethyl ketone solution of a copolymer beforeneutralization (solid content: 50%, acid value: 162.5 mgKOH/gcopolymer).

(2) To 201 parts of the methyl ethyl ketone solution of the copolymerbefore neutralization obtained above was gradually added 100 parts of3.9% aqueous ammonia under stirring to thereby neutralize the carboxylgroups in the copolymer before neutralization. After pH of the reactionmixture became about 7 to 8, the methyl ethyl ketone was removed at atemperature of 50° C. under reduced pressure, thereby obtaining anaqueous solution (pH 7.7) of the copolymer (B) (solid content: 54%)(this copolymer is hereinafter referred to as “aqueous solution of thecopolymer (B1)”.

SYNTHESIS EXAMPLES 7 to 12

Copolymers (B2) and (B3) and copolymers (b1) to (b4) were synthesized inthe same manner as in Synthesis Example 6 except for using the monomersas shown in Table 3 below.

TABLE 3 Properties of aqueous copolymer solution Copolymer beforeCopolymer after neutralization neutralization Base for neutralizationSynthesis Sym- Monomer composition Tg Boiling point Example bol (weightratio) (° C.) Kind (° C.) 6 B1 BMA/MMA/AN/ 88.6 NH₃ −34.4 HEMA/MAA =20/10/35/10/25 7 B2 MMA/ST/AA = 104.9 NH₃ −34.4 85/5/10 8 B3 BMA/MMA/AA= 84.5 NH₃ −34.4 20/50/30 9 b1 AEH/HEM/MAA = −43.6 NH₃ −34.4 75/10/1510  b2 BMA/MAA/AN/ 88.6 TEA¹⁾ 360.0 HEMA/MAA = 20/10/35/10/25 11  b3BMA/BA/MAA = −18.9 NH₃ −34.4 10/65/25 12  b4 INA/HEMA/AA = −56.9 TEA¹⁾360.0 75/10/15 ¹⁾TEA: Triethanol amine.

EXAMPLE 1

(1) 80 parts of the aqueous emulsion (A1) obtained in Synthesis Example1 and 20 parts of the aqueous solution of the copolymer (B1) obtained inSynthesis Example 6 were supplied into a beaker and stirred at roomtemperature (25° C.) for 15 minutes to obtain a liquid adhesivecomposition.

(2) The adhesive composition obtained in the above (1) was applied ontoa polyester film having a thickness of 50 μm with a bar coater such thata dry adhesive layer thickness was 20 to 30 μm, and then dried at 70° C.for 5 minutes to obtain an adhesive sheet.

(3) The results of the measurement of the adhesive force, holding power,probe tack and water resistance of the adhesive sheet obtained in theabove (2) are shown in Table 4 below. Examples 2 to 4 and ComparativeExamples 1 to 5

Each aqueous emulsion (A) and aqueous copolymer solution shown in Table4 were mixed in the proportion as shown in Table 4, and an adhesivecomposition was prepared in the same manner as in Example 1 (1). Usingthe adhesive composition obtained above, an adhesive sheet was producedin the same manner as in Example 1 (2). Adhesive force, holding power,probe tack and water resistance of the sheet were measured. The resultsare shown in Table 4 below.

TABLE 4 Physical properties of adhesive sheet Adhesive force (g/25 mm)Holding power Composition of Before After heating Before After heatingadhesive layer heating 100° C. 140° C. heating 100° C. 140° C. Example(weight ratio) (25° C.) 2 min 2 min (25°) 2 min 2 min Example 1 A1/B1 =80/20 3 or less 100 1100 0 min 0.0 mm 0.0 mm Example 2 A2/B2 = 80/20 3or less 3 or less  850 0 min  0 min 0.0 mm Example 3 A3/B3 = 80/20 3 orless  15 1200 0 min 0.0 mm 0.0 mm Example 4 A4/B1 = 80/20 3 or less 1501050 0 min 0.0 mm 0.0 mm Comparative A5/b1 = 80/20 1150  1150  1140 10min  60 min 60 min Example 1 Comparative A1/b2 = 80/20 3 or less 1101120 0 min 0.0 mm 0.0 mm Example 2 Comparative A1/b3 = 80/20 910 930 920 10 min  40 min 45 min Example 3 Comparative A1/b4 = 80/20 950 970 970 15 min  20 min 25 min Example 4 Comparative A1²⁾ 900 900  910 0.2mm 0.2 mm 0.2 mm Example 5

1) Kinds and mixing ratio of the aqueous emulsion (A) and the aqueoussolution of the copolymer (B) or (b) in each adhesive composition.

2) Aqueous emulsion (A1) alone (containing sodium dodecylbenzenesulfonate).

It is seen from the data shown in Table 4 above that the adhesive sheetsof the Examples 1 to 4, which had each an adhesive layer composed of anaqueous emulsion (A) containing an alkyl (meth)acrylate-based polymer(a) having a glass transition temperature of −10° C. or below obtainedby emulsion-polymerizing radically polymerizable monomers containing asthe main component an alkyl (meth)acrylate in an aqueous medium, and acopolymer (B) having a glass transition temperature of 20° C. or aboveobtained by copolymerizing an α,β-ethylenically unsaturated carboxylicacid with other radically polymerizable monomers, wherein a part or theentire amount of the carboxyl groups in the copolymer had beenneutralized with a base having a boiling point of 110° C. or below, havean adhesive force at a temperature of 25° C. determined by the 180°peeling test method as specified in JIS Z 0237 of 10 g/25 mm or less,and do not show adhesive properties at ordinary temperature but, whenheated, exhibit good adhesive properties, which enable the adhesivesheets to effectively use as heat-sensitive adhesive sheets.

In contrast to this, the adhesive sheets of Comparative Examples 1, 3and 4, that is, adhesive sheets having an adhesive layer composed of anaqueous emulsion (A) and an aqueous copolymer having Tg lower than 20°C., already have high adhesive properties even at ordinary temperatureand thus do not have heat-sensitive characteristics.

Further, it is seen from the data of Comparative Examples 2 and 4 thatif the aqueous copolymers obtained by neutralizing the carboxyl groupswith a base having a boiling point exceeding 110° C. are used, waterresistance of the adhesive sheets becomes poor.

The data of Comparative Example 5 indicate that in the adhesive tapehaving an adhesive layer made of the aqueous emulsion (A) alone, theadhesive layer shows adhesive properties even at ordinary temperatureand the adhesive properties do not basically change before and afterheating.

EXAMPLE 5

(1) A monomer mixture was prepared by mixing 95 parts of 2-ethylhexylacrylate and 5 parts of 2-hydroxyethyl methacrylate.

(2) 100 parts of deionized water and 30 parts of the aqueous solution ofthe copolymer (B1) prepared in Synthesis Example 6 were added to a flaskequipped with a stirrer, a thermometer, a condenser and a nitrogen inlettube. After rising temperature to 60° C. in a nitrogen atmosphere, 5parts of an aqueous solution of t-butyl hydropheroxide, 5 parts of a 10%aqueous solution of RONGALITE (trade name: formaldehyde sodiumsulfoxylate dihydrate) and 100 parts of the monomer mixture prepared inthe above (1) were added dropwise over 3 hours. After completion of thedropwise addition, reaction was continued for additional 2 hours at thesame temperature to complete the polymerization, thereby obtaining anaqueous polymer emulsion having a solid content concentration of 49%.The contents of the aqueous polymer emulsion obtained are shown in Table5 below.

(3) The aqueous polymer emulsion obtained in the above (2) was appliedonto a polyester film having a thickness of 50 μm with a bar coater at adry adhesive layer thickness of 20 to 30 μm and then dried at 70° C. for5 minutes to prepare an adhesive sheet.

(4) The results of the measurement of the adhesive force, holding power,probe tack and water resistance of the adhesive sheet obtained in theabove (3) are shown in Table 6 below.

EXAMPLE 6

A monomer mixture was prepared by mixing 25 parts of butyl acrylate, 70parts of 2-ethylhexyl acrylate, 4.9 parts of acrylonitrile and 0.1 partof N-methylolacrylamide.

The monomer mixture prepared above was mixed with 30 parts of theaqueous solution of the copolymer (B2) prepared in Synthesis Example 7and 40 parts of deionized water. The resulting mixture was emulsifiedwith a three-one mortar to prepare an aqueous emulsified dispersion.

Polymerization was conducted in the same manner as in Example 5 usingthe aqueous emulsified dispersion obtained above to prepare an aqueouspolymer emulsion with a solid content concentration of about 49%. Thecontents of the aqueous polymer emulsion obtained are shown in Table 5.An adhesive sheet was produced using the aqueous polymer emulsionobtained, and the adhesive force, holding power, probe tack and waterresistance thereof were measured by the above-mentioned methods. Theresults are shown in Table 6.

EXAMPLE 7

A monomer mixture was prepared by mixing 35 parts of ethyl acrylate, 60parts of butyl acrylate, 4.7 parts of methyl methacrylate and 0.3 partsof N-methylolacrylamide.

Emulsion polymerization was conducted in the same manner as in Example 5except for using, in the production of the aqueous polymer emulsion, 20parts of the aqueous solution of the copolymer (B3) prepared inSynthesis Example 8 and 100 parts of the monomer mixture as describedabove, to prepare an aqueous polymer emulsion as shown in Table 4. Theadhesive force, holding power, probe tack and water resistance weremeasured for an adhesive sheet produced by using the above aqueouspolymer emulsion. The results are shown in Table 6.

EXAMPLE 8

A monomer mixture was prepared by mixing 95 parts of butyl acrylate, 4parts of acrylonitrile and 1 part of acrylic acid.

Emulsion polymerization was conducted in the same manner as in Example 5except for using, in the production of the aqueous polymer emulsion, 30parts of the aqueous solution of the above-mentioned copolymer (B1) and100 parts of the monomer mixture as described above to prepare anaqueous polymer emulsion as shown in Table 4. The adhesive force,holding power, probe tack and water resistance were measured for anadhesive sheet produced by using the aqueous polymer emulsion asdescribed above. The results are shown in Table 6.

COMPARATIVE EXAMPLES 6 to 9

Aqueous polymer emulsions were prepared in the same manner as in Example5 except for using 100 parts of monomer mixtures as shown in Table 5 asthe radically polymerizable monomer mixture containing an alkyl(meth)acrylate-based polymer as the main component and 30 parts of thecopolymer aqueous solutions as shown in Table 5.

Adhesive sheets were produced using the aqueous polymer emulsionsobtained and physical properties thereof were evaluated. The results areshown in Table 6.

TABLE 5 Contents of aqueous polymer emulsion Aqueous Aqueous polymeremulsion Exam- copolymer Composition of radically polymerizable Tg(°C.)²⁾ ple. solution monomers¹⁾ (wt. ratio) (calcd.) Exam- B1 AEH/HEMA =95/5 −66.1 ple 5 Exam- B2 BA/AEH/AN/NMAM = 25/70/4.9/0.1 −61.6 ple 6Exam- B3 EA/BA/MMA/NMAM = 35/60/4.7/0.3 −40.6 ple 7 Exam- B1 BA/AN/AA =95/4/1 −51.2 ple 8 Com- b1 AEH/HEMA = 95/5 −66. 1 parative Exam- ple 6Com- b2 AEH/HEMA = 95/5 −66.1 parative Exam- ple 7 Com- b3 AEH/HEMA =95/5 −66.1 parative Exam- ple 8 Com- b4 AEH/HEMA = 95/5 −66.1 parativeExam- ple 9 ¹⁾Composition of radically polymerizable monomers containingalkyl (meth)acrylate as the main component. ²⁾Tg (calculated) of alkyl(meth)acrylate-based polymer.

TABLE 6 Physical properties of adhesive sheet Adhesive force (g/25 mm)Holding power Before After heating Before After heating heating 100° C.140° C. heating 100° C. 140° C. Example (25° C.) 2 min 2 min (25°) 2 min2 min Example 5 3 or less 100 1200 0 min 0.0 mm 0.0 mm Example 6 3 orless 3 or less  830 0 min  0 min 0.2 mm Example 7 3 or less  10  500 0min 0.0 mm 0.0 mm Example 8 3 or less 150 1250 0 min 0.0 mm 0.0 mmComparative 1100 1100  1140 50 min  60 min 60 min Example 6 Comparative3 or less 110 1120 0 min 0.2 mm 0.3 mm Example 7 Comparative 1000 1000 1000 30 min  40 min 40 min Example 8 Comparative  900 920  920 20 min 20 min 20 min Example 9 Physical properties of adhesive sheet Probe tackWater resistance Before After heating Before After heating heating 100°C. 140° C. heating 100° C. 140° C. Example (25° C.) 2 min 2 min (25°) 2min 2 min Example 5 5 or less  55 950 ◯ ◯ ◯ Example 6 5 or less 5 orless 700 ◯ ◯ ◯ Example 7 5 or less  28 450 ◯ ◯ ◯ Example 8 5 or less  70 80 ◯ ◯ ◯ Comparative 900 900 920 ◯ ◯ ◯ Example 6 Comparative 5 or less 60 940 X X X Example 7 Comparative 815 850 880 ◯ ◯ ◯ Example 8Comparative 780 880 810 X X X Example 9

Industrial Applicability

The heat-sensitive adhesive sheets of the present invention do not showadhesive properties or do not substantially show adhesive properties, atordinary temperature. Therefore, it is not necessary to subject anadhesive layer to a release treatment, such as use of a release paper orcoating a release agent on a substrate surface, and the sheets can bestored, distributed and sold with or without packaging. As a result, thesheets are excellent in the points of resource saving, simplification ofproduction process and cost reduction.

The heat-sensitive adhesive sheets of the present invention can exhibitvarious adhesion performances of from high adhesiveness to moderateadhesiveness, or in some cases, to slighly low adhesiveness, by heatingand/or applying pressure. Therefore, the sheets can be used in wide usessuch as labels, tapes, seals for packages, wallpapers, markers, andothers.

In particular, where it is used as a wallpaper, use method becomespossible that the wallpaper is adhered and fixed by conductingpositioning utilizing low adhesiveness and then applying heat.

Further, even if the heat-sensitive sheet of the present invention isadhered to an adherend by exhibiting adhesiveness by heating and/orapplying pressure and temperature returns to ordinary temperature, theadhesive properties once exhibited are maintained, so that theheat-sensitive adhesive sheet adhered does not peel from the adherend.

Where the heat-sensitive adhesive sheet of the present invention adheredto the adherend is desired to peel from the adherend, it can be easilypeeled off therefrom by merely pulling with fingers. The adhesive layerthus peeled can again be adhered to the same adherend or anotheradherend, if necessary.

Further, the heat-sensitive adhesive sheet of the present invention canfreely set the heating temperature for exhibiting adhesive properties,and the adhesive layer does not become hard even after the passage oftime. Therefore, there is no fear that the adherend peels off when theadhesive sheet is bent. In addition, trouble does not occur that thecomponents of the adhesive layer ooze out onto the substrate.

What is claimed is:
 1. A heat-sensitive adhesive sheet, characterized inthat it is a heat-sensitive adhesive sheet having on a substrate anadhesive layer comprising a polymer composition emulsion (C) obtained byemulsion polymerizing a radically polymerizable monomer comprising as amain component, an alkyl (meth)acrylate forming an alkyl(meth)acrylate-based polymer (a) having a glass transition temperatureof −10° C. or less, in an aqueous medium in the presence of awater-soluble or water-dispersible copolymer (B) having a glasstransition temperature of 20° C. or more, which is a copolymer having anacid value of 30-260 mgKOH/g obtained by copolymerizing anα,β-ethylenically unsaturated carboxylic acid with other radicallypolymerizable monomers, wherein a part or the entire amount of carboxylgroups in the copolymer is neutralized with a base having a boilingpoint of 110° C. or less; and that an adhesive force determined by the180° peeling test method as specified in JIS Z 0237 at a temperature of25° C. is 10 g/25 mm or less, and wherein the amount of carboxyl groupsneutralized in the copolymer (B) is at least 30 mol % of the carboxylgroups.
 2. The heat-sensitive adhesive sheet as claimed in claim 1,wherein the amount of the alkyl (meth)acrylate in saidradical-polymerizable monomers used for obtaining said alkyl(meth)acrylate type polymer (a) is 60 to 100% by weight based on thetotal amount of the radically polymerizable monomers.
 3. Theheat-sensitive adhesive sheet as claimed in claim 1, wherein the alkylgroup in said alkyl (meth)acrylate in said alkyl (meth)acrylate-basedpolymer (a) has 1 to 9 carbon atoms.
 4. The heat-sensitive adhesivesheet as claimed in claim 1, wherein the alkyl (meth)acrylate-basedpolymer (a) in said alkyl (meth)acryalte-based polymer (a) has a glasstransition temperature of −30° C. or below.
 5. The heat-sensitiveadhesive sheet as claimed in claim 1, wherein the α,β-ethylenicallyunsaturated carboxylic acid in said copolymer (B) is selected from thegroup consisting of acrylic acid, methacrylic acid, crotonic acid,maleic acid, fumaric acid, itaconic acid, citraconic acid and maleicanhydride.
 6. The heat-sensitive adhesive sheet as claimed in claim 1,wherein the radically polymerizable monomer to be copolymerized with theα,β-ethylenically unsaturated carboxylic acid in said copolymer (B) isselected from the group consisting of (meth)acrylates, aromatic vinylcompounds, (meth)acrylonitrile, vinyl acetate, vinyl propionate,N-methylol (meth)acrylamide, vinyl chloride, vinyl fluoride, vinylidenechloride, vinylidene fluoride, ethylene and trichloroethylene.
 7. Theheat-sensitive adhesive sheet as claimed in claim 1, wherein saidcopolymer (B) has a glass transition temperature of from 30 to 200° C.8. The heat-sensitive adhesive sheet as claimed in claim 1, wherein theratio of said copolymer (B) (solid content) to said alkyl(meth)acrylate-based polymer (a) (solid content) is copolymer (B)/alkyl(meth)acrylate-based polymer (a)=5/95 to 80/20 by weight ratio.